meson: Make shared-llvm into a tri-state boolean

Choosing LLVM's link mode is legitimate on UNIX systems, but only static
actually really works under Windows.

Give shared-llvm a default 'auto' mode which will pick the previous
default of true (shared) on UNIX systems, but newly defaulting to false
(static) on Windows.

Signed-off-by: Daniel Stone <daniels@collabora.com>
Suggested-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/4555>
diff --git a/meson.build b/meson.build
index cb275e7..6b85c95 100644
--- a/meson.build
+++ b/meson.build
@@ -1401,6 +1401,11 @@
 endif
 
 _shared_llvm = get_option('shared-llvm')
+if _shared_llvm == 'auto'
+  _shared_llvm = (host_machine.system() != 'windows')
+else
+  _shared_llvm = (_shared_llvm == 'true')
+endif
 _llvm = get_option('llvm')
 
 # The cmake method will never find libllvm.so|dylib; this is fine for windows